The present invention pertains to inflatable packers which are used to seal across downhole conduits, such as well casings. A typical inflatable packer includes a tubular, inflatable packer body which usually has an outer elastomeric layer and an inner reinforcing layer of flexible but non-elastomeric material, e.g. braided metal cords. Rigid tubular packer heads are coaxially connected to respective opposite ends of the packer body. A tubular mandrel is disposed within the packer heads and packer body. Usually, the upper head is fixed with respect to the mandrel, while the lower head may be longitudinally slidable thereon, to accommodate radial expansion when the packer is inflated.
Although the packer body comprises an elastomeric layer, it is the elastomeric sleeve-like bladder, positioned between the packer body and the mandrel, which actually defines the chamber into which fluid is pumped to inflate the packer, and which retains that fluid. The upper packer head typically includes passageways or other means for introducing inflation fluid to the annular space between the mandrel and the bladder and then sealing it closed.
Particular problems are experienced with inflatable packers having high expansion ratios. For example, a packer which must be run into the well through a string of relatively small tubing, and after emerging therefrom, seal against a larger well casing, must sometimes expand from a relaxed outer diameter to a sealing outer diameter by an amount in excess of 200%. Under such conditions, at least two factors may lead to failure of the bladder and consequent leakage of the packer.
The first factor is a function of the limitations on the elongation ability of elastomers. This ability to substantially reduced at the high temperatures experienced downhole, i.e. above about 150.degree. F.. For example, a nitril rubber with elongation characteristics of 700% to failure at ambient temperature will be reduced to 250% at 210.degree. F.
Secondly, as the packer is inflated, particularly when a rather large increase in outer diameter is effected, the metallic or other members making up the reinforcing layer of tha packer body tend to separate and form gaps in that layer. Meanwhile, the expanding bladder is becoming thinner and thinner, and tends to extrude through any such gaps. This is a particular problem near the packer heads where the packer body must form corners as it is inflated. Again, any portions of the bladder extruding through such gaps may fail, again allowing the packer to leak.
The two problems discussed above also tend to aggravate each other. For example, a portion of a bladder extruding through a gap in the reinforcing layer of the packer body is more likely to fail under high elongation conditions, as described above, and particularly at high temperatures.
Furthermore, these problems often do not occur immediately after inflation of a packer, but rather, after it has been inflated and in use for some time. This has been shown by tests of conventional packers tested in boiling water. Such packers may withstand the test for as much as two or three hours. In the past, one might have conducted such a test, assumed tha packer was reliable, and placed it in use, only to find that it would fail after a longer period of time.